1. Field of the Invention
The invention relates to a metal gate structure and a manufacturing method thereof, and more particularly, to a metal gate structure and a manufacturing method thereof integrating a gate last process.
2. Description of the Prior Art
With the trend towards scaling down the size of the semiconductor devices, conventional methods, which are used to achieve optimization, such as reducing thickness of the gate dielectric layer, like reducing the thickness of silicon dioxide layer for example, face problems such as current leakage due to the tunneling effect. In order to keep progressing to the next generation products, high dielectric constant (hereinafter abbreviated as high-k) materials are used to replace the conventional silicon oxide or oxynitride layer to serve as the gate dielectric layer, because they can decrease the physical limit thickness effectively, reduce the current leakage, and achieve equivalent capacities for identical equivalent oxide thicknesses (EOT).
On the other hand, the conventional polysilicon gates also face problems such as inferior performances due to boron penetration and unavoidable depletion effect, which increases the equivalent thickness of the gate dielectric layer, reduces the gate capacitance, and decrease a driving force of the devices. Thus work function metals are developed to replace the conventional polysilicon gates to be the control electrodes that are suitable to be the high-K gate dielectric layer.
However, there is always a continuous need in the semiconductor processing art to develop semiconductor device that render superior performances and reliability, such as ensuring that the metal gate of the P-type MOS (PMOS) has a work function of about 1.9 eV to 5.2 eV when the EOT of the high-K gate dielectric layer is reduced, even though the conventional silicon dioxide or silicon oxynitride gate dielectric layer is replaced by a high-K gate dielectric layer and the conventional polysilicon gate is replaced by a metal gate.